Most fine structured devices such as integrated circuit chips, microelectromechanical devices, etc. are made of multiple layers of precisely aligned circuit or mechanical patterns. The patterns are usually formed through multiple high precision lithography steps during manufacturing where most patterns are required to be aligned precisely with respect to each other. However, even with best systems and efforts, some amount of lateral misalignment between different patterns is unavoidable. Overlay error is the lateral misalignment between different patterns or targets.
Traditionally, overlay error has referred to the alignment error between successive device layers, which will be called between-layer overlay error herein. However, in some cases such as double- or multi-patterning lithography, overlay error may refer to the lateral misalignment between different patterns in the same layer. This will be called single- or within-layer overlay error herein.
Currently, controlling overlay error is one of the most difficult tasks in semiconductor manufacturing because of the ever shrinking design rules and the complexity in modern manufacturing processes. Because overlay error can affect yield, device performance and reliability, it must be measured precisely. Overlay error can be measured in many different ways. However, in most cases, overlay error is measured optically by capturing the image of specially designed alignment marks called overlay targets and processing the image with a computer. Optical measurement is preferred because it is non-destructive and fast.
The overlay we are interested in is in the functional pattern areas. Unfortunately, optical overlay measurement systems can rarely measure the overlay of functional patterns directly because most of the functional patterns are too fine to be resolved by optical systems. Optical overlay measurement systems usually measure the overlay of functional patterns indirectly using a special non-functional pattern called an overlay target or simply a target. Overlay targets are usually made much coarser than the functional patterns in order to be resolved by optical systems. Optical overlay measurement systems measure the overlay error in the target area and make the assumption that the overlay error in the functional pattern area is the same or at least well correlated with the overlay error in the target area.
Because of the indirectness of the measurement, it is very important to have a good correlation between the two overlay measurements, the functional pattern overlay and the target overlay. In order to have a good correlation between the two overlays, we need to bind the overlay targets to the functional patterns tightly. Their tight binding is commonly achieved by including overlay targets as a part of the pattern design and by printing both the functional patterns and the targets at the same time. This kind of same time design and printing of both functional patterns and targets assures a good correlation between the overlay error measured in the functional pattern area and the overlay error measured in the target area.
When targets are printed, targets belonging to different process layers are printed in the same area on wafer in order to facilitate an accurate measurement of overlay errors. The imaging system takes a picture of all the individual targets in a single image. Thus, an overlay target viewed by an overlay measurement system is not a single target but a group of individual targets, which will be called a target set herein. Thus, a target set is different from an individual target. However, a target set will also be called a target herein whenever the context makes its meaning clear. The measured overlay error is the lateral offset or misalignment between different individual targets printed in the same area. In order to facilitate an accurate overlay measurement, the individual targets are usually printed with no or little overlap with each other even if they are placed in the same area. Thus, all individual targets are usually well distinguishable in the image. A target set usually contains two individual targets. However, it can contain more than two individual targets. In an example, the target area is 100 μm2 or less.
Currently, two optical technologies are being used for optical overlay measurement. One is an image-based technology and the other is a diffraction-based technology. Image-based technology takes an image of a target set and processes the image based on intensity slopes to determine the overlay error. Diffraction-based technology uses multiple targets each of which is made of two interleaving gratings. It does not resolve individual grating lines but measures the variation of diffraction efficiency caused by the offset variation between the interleaved gratings. The variation of offset between the two interleaved gratings is the overlay error that needs to be determined.
Diffraction-based overlay measurement systems were developed to achieve higher measurement accuracy than image-based systems. However, these systems have several drawbacks such as a requirement for multiple targets, inflexible target design, complicated system calibration, higher cost, etc. Especially, multiple diffraction targets require a relatively large clear area on the wafer to print all the targets. This makes this technology difficult to employ in many important applications such as in-die overlay measurement where large clear areas for target printing are not available.
The existing image-based overlay measurement systems are reliable and robust. However, they have many critical drawbacks such as requiring large targets, poor handling of weak overlay signals, shallow depth of focus, severe influence from patterns around the target and also from imaging system aberrations, etc. Therefore, the existing image-based overlay measurement systems are not suitable for the future applications most of which require small targets, good handling of weak signals, good filtering of detrimental interferences or influences, etc.